Activation of nuclear factor κB (NF-κB) transcription factors [for example, in response to tumor necrosis factor–α (TNF-α) receptor (TNFR) signaling] depends on activation of inhibitor of κB (IκB) kinase β (IKKβ), which phosphorylates IκB, leading to its degradation. NF-κB proteins are then released and translocate to the nucleus to activate the transcription of target genes. IKKβ constitutes part of a complex containing IKKα and NEMO (also known as IKKγ), which is an adaptor protein that binds to ubiquitin chains assembled at receptors such as activated TNFRs, so that IKKβ can be activated. Noting studies that have implicated IKKβ in other signaling pathways, including the DNA damage response and autophagy, Schröfelbauer et al. investigated how IKKβ is targeted toward stimulus-specific substrates. Experiments in wild-type and NEMO-deficient cells showed that a constitutively active form of IKKβ (IKKβEE) required NEMO to phosphorylate IκB and activate NF-κB. Reconstitution of NEMO-deficient cells with wild-type or mutant forms of NEMO showed that the C-terminal zinc finger (ZF) of NEMO was required for NF-κB activation in the presence of IKKβEE. Coimmunoprecipitation studies showed that NEMO bound to IκB through the ZF region and that this interaction was increased in the presence of IKKβ, suggesting formation of a ternary complex. Expression of mutant NEMO that enabled IKKβ activation but failed to bind to IκB resulted in the enhanced phosphorylation of alternative IKKβ substrates and enhanced activation of autophagy, which does not require NF-κB activation. Together, these data suggest that in addition to being required for the activation of IKKβ, NEMO acts as a scaffold protein to direct IKKβ to IκB and ensure NF-κB activation.
B. Schröfelbauer, S. Polley, M. Behar, G. Ghosh, A. Hoffmann, NEMO ensures signaling specificity of the pleiotropic IKKβ by directing its kinase activity toward IκBα. Mol. Cell 47, 111–121 (2012). [PubMed]